The present invention concerns the control of a laser source with multiple laser units for the energy and spatial optimization of a laser surface treatment.
It finds a general application in laser surface treatment, such as descaling, cleaning, polishing and preparing a surface. More particularly it finds application in the annealing, by excimer laser, of layers of amorphous silicon placed on a substrate.
In general terms, the annealing of amorphous silicon by excimer laser consists of raising the temperature of the layer of amorphous silicon very rapidly to the melting point in order to obtain crystallization in the form of a polysilicon. The melting of the layer of silicon must not for all that disturb the standard glass substrate too significantly.
In practice, the heating effect is due to the optical absorption of the laser beam by a thin film of amorphous silicon, for example over a thickness of less than 30 nm.
Annealing amorphous silicon by excimer laser requires excellent knowledge of the structural characteristics of the sample to be treated, as described at least partly in the patent application entitled "Device and method of controlling laser surface treatment", filed in France by the Applicant, on Aug. 11, 1995, under the number 95 09778, the content of which forms an integral part of the present application for all useful purposes.
First, heating by laser must be effective since it greatly influences the thermal change in the sample, notably with regard to the melting of the layer of silicon, which must be complete, and with regard to the melting of the substrate, which must be avoided.
Secondly, the speed of cooling and solidification of the layer of silicon must be relatively slow since it determines the size of the polysilicon grains present in the layer after annealing, since the large crystals are the most favourable to optimum functioning, for certain microelectronic devices based on silicon.
Applicant has observed that such an optimum treatment, from an energy and spatial point of view, is difficult to obtain using a single laser source.
Applicant therefore, proposes to use a laser source comprising at least two laser units.
However, the control of a laser source with several units, in combination with the laser surface treatment is, in its turn, difficult to implement.
The present invention affords a solution to this problem.